Radio communication system



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Attorney Patented Feb. 22, 1949 RADIO COMMUNICATION SYSTEM Prafulla Kumar Chatterjea and Leslie Wilfred England, assignors, by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a cor- Houghton,

poration of Delaware London,

Application May 7, 1943, Serial No. 486,038 In Great Britain May 7, 1942 Claims. (01. 250 -17) In certain circumstances, it is imperative that intelligible radio signals be transmitted in spite of powerful jamming by a nearby transmitter.

The normal method of transmission employed in the long wave, medium wave and short Wave ranges consists of the.amp1itude modulation of a single carrier thus utilising only a narrow band of frequencies above and below this carrier frequency so that effective jamming of, or interference with, this type of transmission is obtained by another transmissiontuned to the same or approximately the same carrier frequency and suitably modulated.

, One such set of circumstances may be as follows:

In a convoy system of naval transport stray vessels may be sighted by an enemy bomber. This bomber then sends information to its base station while continuing to jam the stray ships transmission. The ship can by use of the present invention transmit emergency information to more than one receiving station and on several carrier frequencies.

, The object of this invention is to provide radio communication arrangements whereby when the transmission from a particular transmitter isjammed or interfered with by a nearby transmitter of sufficient strength to completely mask the transmission of the former on its initial, normal wavelength, then the' signal which is being transmitted is automatically continued on one or more pre-arranged carrier frequencies, preferably a plurality of carrier frequencies, so that continued interference with all of said carrier frequencies then becomes much more difficult and the signal may be obtained on one or more of the carrier frequencies.

Radio communication arrangements according to the present invention comprise arrangements herein called the normal arrangements for transmitting the signals as a modulation of a carrier frequency herein called the normal carrier frequency, emergency arrangements for transmitting the signals as a modulation of a plurality of carrler frequencies dispersed over a wide frequency band, a device responsive to the reception of radio waves at substantially the normal carrier frequency from a source other than the said transmitter, and a switching arrangement adapted to be operated by said device in response to said reception to switch over from the normal arrangements of transmission on a single carrier frequency to the emergency frequencies. Pulse modulation systems are well suited as a source of a'plurality of carrier frequencies. It is well known that a pulse transmission can be received on the carrier frequency employed and at frequencies difiering therefrom by the pulse repetition frequency or by any multiple of that frequency. Also in the case of duration modulated pulses, that is Where the duration of the pulses varies in accordance with the instantaneous amplitude of the signal wave, or in the case of time modulated pulses, that is where pulses of constant duration occur at time intervals which varyin accordance with the instantaneous amplitude of the signal wave, the signals may be received directly on a receiver designed for the reception of amplitude modulatedwaves.

Arrangements for obtaining the plurality of carrier frequencies, each amplitude modulated with the required signal are described in the specifications of copending applications, Ser. No. 375,814, filed January 24, 1940, and Ser. No. 488,572, filed December 10, 1942, which latter ap plication is now abandoned and are used. alternatively in conjunction with an amplitude modulated normal carrier transmitting system. The modulation may also be a frequency or phase modulation of a carrier frequency. In this-case the emergency arrangements comprise a pulse modulation system in which the pulses consist of trains of carrier frequency waves, the frequency or phase of the carrier wave in each train being representative of the instantaneous amplitude of the signal wave. The receiver must of course be such as to receive phase or frequency modulated transmissions. In the specification of application Ser. No. 488,572 now abandoned aforementioned, there is described a system in which the pulses comprise trains of carrier frequency which are frequency or phase modulated in accordance with the instantaneous amplitude of .the signal wave, and the trains of waves are also amplitude modulated in accordance with the instantaneous amplitude of the signal wave. By this means the signal wave can be'received by receivers de-' signed for frequency or phase modulation and can be received by receivers designed only for amplitude modulation. Damped trains of amplitude. modulated waves may be produced by utilising an oscillating circuit which is set into oscillation by pulses of constant duration the slope of whose leading and trailin edges varies in accordance with the instantaneous amplitude of the signal wave. The' initial amplitudes of the damped trains set up depends upon the slopes of the edges of the exciting pulses. The slope of the edges of the constant duration pulses are l I I 3 varied in accordance with the instantaneous amplitude of the signal wave.

Application Ser. No. 375,814 describes an arrangement in which the fixed edge: that is, the edge of a pulse which occurs atconstant fixed intervals of time, is time modulated toavery small" modulation depth in accordance with the instantaneous amplitude of the signal wave. type of pulse modulation has been termed a flute ter pulse modulation and is in fact a very small duration modulation which can be applied'to pulses of short constant duration; This, flutter modulation may be used at the transmitter or receiver to set up damped trains of waves varying in initial amplitude in accordance with the original signal amplitude. The said application de. scribes means for obtaining an amplitude modulation from the flutter pulse modulation by shock exciting a tuned circuit and allowing only the waves due to the trailing edge of the pulse to pass into an output circuit.

In all these and other pulse systems, the modulations may be received on several frequencies, greater or less than, and differing from, the principal carrier'frequency (when each pulse comprises a train of carrier frequency) or from the pulse repetition frequency (when direct current pulses are employed) by a multiple of the pulse repetition frequency.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings including Figures' 1-3 which show diagrammatically several practical arrangements embodying the invention. Each of the figures illustrates a pulse generator whose pulse recurring frequency can be controlled as shown by the arrows.

Referring to Figure l, the numeral I represents a transmitter, modulated by unit 2' and feeding transmitting aerial array TAI. Unit 3 is a special pulse generating unit having means for controlling the pulse recurrence frequency as shown by the arrow, which is also modulated by unit 2. Unit 4 is a low pass filter unit designed to thenormal modulation frequency band: and the output from this is caused to operate the switching arrangement 5. Under normal operation, and using amplitude modulation contacts 6 and 'l of switch 5 are connected together so causing unit 2 to modulate I, while switches 33 and 34 are open and the resulting signal is then radiated by TAI. If frequency modulation is desired, switches 34 and 33 are closed manually. If now another nearby transmitter is tuned on to the same carrier frequency as that on which I is working, its signal will be picked up by TAI and a beat note will result between the tWo carriers or between the modulations of the two carriers which will pass through unit 4 and if of large enough amplitude, will operate switch 5, connecting terminals 6 and 8 and so causing transmitter I to be modulated by the pulse generator 3 with resulting series of, signal modulated carrier frequencies which will occur above and below the normal operating frequency of I spaced out at intervals equivalent to the pulse generators repetition frequency; The operation is quite automatic and the receiving station, on hearing the interfering signal, has only to be tuned to one of the pre-arranged alternative frequencies.

Arrangements will be described for switching the transmitter for conversion from a normal single carrier radiation to a multiple carrier radiation. Y

This

lated by unit 22.

Fig. 2 shows schematically an arrangement in which only one oscillator is used. The automatic switching apparatus is shown as low pass filter 4 with control.

Under normal operation switch 23, is open and 24 and 25 closed. I8 represents the main oscillator, I9 the buffer and modulator stage modu- The output from I9 is then connected directly to the main amplifier 2| and then to aerial array TA2I. Unit I! is a pulse generator, time or. duration modulated by unit 22: andunit 20 isa band pass filter. Under emergency conditions switch 23 is closed, 24 and 25 opened so that the output from IT modulates oscillator I8 direct producing a, multiplicity of amplitude, modulated carrier frequencies which after passing through I9, which functions as an amplifier, are passed through filter 20 when the appropriate number are selected before amplification by 2! and subsequent radiation by TA2I.

Another arrangement which is shown diagrammatically in Fig. 3 makes use of a different method of producing the requisite emergency signals. Under normal conditions switch 32is connected to34 and switch 36 to 33 so that the oscillation produced at 26 is amplitude modulated at 27 by the signal output of 29 and then amplifled by 28 before radiation at aerial TA28, as with previous arrangements. Under emergency conditions switch 32 is connected to 35and switch 36- to 31 so that the output from oscillator 26 is fed to a unit 30 whose function is to convert this continuous oscillation intothe amplitude modulated damped trains which consist of the required modulated carriers of the correct frequency separation. Unit 30 may comprise arrangements as described in the applications Ser, No, 375,814 and 88,572 which latter application is now aban doned. The number of carriers required arethen selected by filter 3| and after amplification by 28 are radiated by aerial TAZB. It is found that the contemporaneous amplitudes of these carriers vary when initially generated so that if a large number of carriers is required and if desired, an additional amplifier may be added, or the characteristic of amplifier 28 may be such as to cause the contemporaneous amplitudes of all carriers to be equal when actually radiated. It is of course essential that the frequency characteristic of amplifier 28: be capable of handling all the required emergency carrier frequencies.

As described in; the copending application Ser. No. 488,572, now abandoned, the principle of operation of unit 30 isthat it must generate a; short duration pulse having a repetition frequency which is an, exact sub-multiple of the oscillation frequency generated by unit 26, the initial edge of the pulse being correctly and constantly phased relative to the particular cycle of the oscillator frequency at which is occurs; The duration of this pulse has then to be modulated by unit 29 and the resulting pulse used to control an amplifier into which the oscillator from 26 isfed.

It is not essential" that on transferring, from 1e normal transmission to the emergency trans! mission the frequencies should be constant but may be varied either automatically or manually in steps, so that other receivers tuned to;a nearby frequency will be more readily contacted; One method of ensuring that an emergency signal is received by all receivers within range; if tuned to frequencies within the emergency signals range is obtained by varying the rate of frequency change'of the" individualparrlers; so

that each sweeps through its frequency range, conforming to dots and dashes of normal code signalling. This effect is readily obtainable by appropriate variation of the pulse repetition frequency used in generating the initial pulse trains.

Whilst some pulse systems have been referred to as providing a plurality of carrier waves other suitable ones will occur to those skilled in the art. Furthermore, the invention is not limited to a plurality of carriers provided'by a pulse transmission system. The carriers may be generated separately and individually, or they may be harmonics of a fundamental carrier frequency.

What is claimed is:

1. Radio communication apparatus comprising normal transmitting means for transmitting signals as a modulation of a normal carrier frequency, emergency transmitting means for transmitting said signals as a modulation of a plurality of carrier frequencies, a device responsive to reception of radio waves at substantially the normal carrier frequency from a source other than said normal transmitting means, and switching means adapted to be operated by said device in response to said reception to switch over from said normal transmitting means to said emergency transmitting means.

2. Radio communication apparatus as claimed in claim 1, in which the emergency transmitting means is arranged to transmit a plurality of carrier frequencies simultaneously modulated by the signal and produced by the modulation of the normal carrier frequency.

3. Radio communication apparatus as claimed in claim 1, in which the modulation of the emergency transmitter means is an amplitude modulation.

4. Radio communication apparatus comprising transmitting means for transmitting signals as a modulation of a single carrier frequency, a pulse modulation transmitter, a device responsive to reception of radio waves at substantially the said carrier frequency from a source other than said transmitting means, and switching means adapted to be operated by said device in response to said reception to switch over from said single carrier frequency transmitting means to said pulse modulation transmitter.

5. Radio communication apparatus as claimed in claim 4, in which the modulation of the pulse reception of radio waves comprises a low pass filter which passes low frequency beat notes producedbetween the transmitted signal and those received from the other source.

9. Radio communication apparatus as claimed in claim 1, in which the emergency transmitter produces a plurality of carrier frequencies and in which these frequencies are so amplified that all carrier frequencies radiate with equal power.

10. Radio communication apparatus as claimed in claim 4, in which means is provided to vary the pulse repetition rate of the pulse modulation transmitter in a predetermined manner.

PRAFULLA KUMAR CHAT'I'ERJEA. LESLIE WILFRED HOUGI-ITON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name I Date 1,750,668 Green Mar. 18, 1930 2,154,923 Walter Apr. 18, 1939 2,209,273 Hills July 23, 1940 

